Devices using magnets for attaching to metallic surfaces are known. Such devices are used in rope access systems and are of particular use as positioning aids to hold a rope close to a surface. For example, when working on the outside of a ship's hull, if access is to be gained by rope from the deck level, because of the shape of the hull it is often necessary to provide some means to secure the rope or a person on the rope to the hull, otherwise the person on the rope is situated too far from the hull to be able to work on it. Another example of the use of such devices is where a person is using rope access techniques to work on a metallic wall, for example the leg of an oil rig. By using a magnetic positioning aid, the worker can secure himself and/or his ropes to the wall on which he is working, thereby limiting the scope for movement of himself with respect to the wall which might result from movement of the object of which the wall forms a part or gusts of wind. Another reason to use a positioning aid is to allow a worker to exert force more effectively. For example, if a worker is hanging from a long rope any force he exerts on the wall will result in him moving away from the wall, which may be undesirable for a particular task. By using a positioning aid, the worker can secure himself with respect to the wall so that when he exerts a force on the wall that force is reacted through the positioning aid, so the worker does not move with respect to the wall.
For a magnetic attachment device to generate sufficient force to hold the weight of a man in position, a significant magnetic force is required. The problem arising from this is that the attachment device must be capable of being removed from the surface, which therefore requires a force greater than the magnetic force to be exerted on the device. The types of task where positioning aids are required to be used often involve applying surface finishes such as paints or sealants. Hence, there is a need for a positioning aid to be removable from a wall without the surface of such a wall being damaged.
One positioning aid that addresses these problems is described in the applicants' United Kingdom patent 2339633. This positioning aid includes a pad mounting magnets, a handle for a user to grip, holes to accommodate ropes or clips and a cam mechanism which is used to lift the magnets off a surface to which they are attached. In order to protect the surface to which the positioning aid is attached, each cam is provided with a flexible protective element, which in use lies between the surface of the cam and the surface to which the positioning aid is attached. Therefore, when the cam is actuated to lift the magnets instead of the cam engaging with for example the paint work of a ship's hull, the cam engages with the flexible element. In this way the paint is protected from damage.
However, the positioning device described in United Kingdom patent 2339633 does suffer from some disadvantages. For example, the flexible elements are relatively flimsy and may become damaged, either through use or even in storage. If the device is then used without flexible element being in the correct position, or not being present, the surface to which the device is attached is likely to be damaged.
The device described in United Kingdom patent 2339633 uses a number of rare earth magnets attached to an iron plate, which serves to distribute the magnetic field. The plate and magnets are then installed in a box and the box welded. Rare earth magnets are hydroscopic. It is therefore imperative that the magnets are sealed from moisture. This is of particular importance given the likely fields of use of such devices (ships, oil rigs and the like). In the device described in GB2339633 the magnets are sealed against ingress of moisture by filling the box housing the magnets with epoxy resin. This construction poses two problems. First, it is difficult to service the device if one of the magnets becomes defective. If a magnet becomes defective it is necessary to break the weld between the iron back plate and the steel box, break the epoxy resin and second, the whole assembly of plate and magnets must be replaced because the epoxy resin cannot reliably be removed from the magnets without damaging them.
Whilst United Kingdom patent 2339633 describes a positioning aid, the same principle may be used for many different purposes. For example 2339633 identifies that the positioning aid may be used as a plate lifter. Such a device may also be used simply as a means to hang another object from. For example, a mechanic working on a vehicle may wish to hang up a light, or hang up some tools.
It would therefore be desirable to provide an improved magnetic attachment device.